Since fiber-reinforced plastics are lightweight and have excellent strength properties, they are widely used in the manufacture of golf club shafts. When playing golf, it is known that lightweight golf clubs, especially lightweight golf club shafts, contribute to significantly increasing swing speed, which in turn brings about a remarkable extension of carry distance derived from an increase in energy generated by acceleration of head speed.
However, while a lightweight golf club shaft accelerates swing speed, it reduces swing stability due to a decrease in the moment of inertia of the golf club, and the ball flight patterns become unstable accordingly. Moreover, a conventional golf club shaft made of fiber-reinforced plastic is set to be lightweight by reducing the thickness of a fiber-reinforced plastic layer, called a straight layer, in which the fiber material is aligned in the longitudinal direction of the shaft, or by reducing the number of straight layers. When such a shaft is used, the flex, an indicator to show the stiffness of a shaft, becomes soft. A shaft with a soft flex may be able to increase the swing speed even with less force. However, for a golfer capable of gaining faster head speed, swing stability at the time of impact may be reduced, thus further negatively affecting ball flight patterns.
Patent Literature 1 discloses a golf club shaft, made to be as lightweight as possible by laminating a fiber-reinforced plastic layer, called a hoop layer, where a fiber material is aligned to be perpendicular to the longitudinal direction of the shaft, and by optimizing its location. However, to achieve as lightweight a shaft as possible, the thickness of a straight layer or the number of straight layers needs to be reduced, thus providing no solution for the above-identified problems.
Patent Literature 2 discloses a golf club having a twist strength enhanced by setting a thickness of no greater than 0.06 mm for a prepreg sheet to be used for a fiber-reinforced plastic layer, called a bias layer, where a fiber material is aligned diagonally to the longitudinal direction of the shaft. However, since a shaft of such a golf club requires two hoop layers to achieve reinforcement in the circumferential direction, it is necessary to reduce the thickness of a straight layer or the number of straight layers so as to obtain a lightweight shaft. Accordingly, no solution is provided for the above-identified problems.
Meanwhile, Patent Literature 3 discloses a lightweight golf club shaft capable of enhancing head speed, formed by arranging the hoop layer only in a region located at 45% of the entire length from the butt end. The head speed is improved by using such a lightweight shaft. However, for a golfer capable of accomplishing faster head speed, a problem of lowered swing stability is not sufficiently solved.